1. Field of the Invention
This invention relates to the surface treatment of ceramic articles for metallization and to a metal layer or a metallized conductor pattern directly and adherently bonded onto a surface of a ceramic substrate, and an improved process for producing the same. More particularly, this invention relates to a printed circuit pattern directly and adherently bonded onto a surface of a ceramic substrate, and an improved process for producing the same.
2. Description of the Prior Art
Metallized conductor patterns or uniform metal layers on ceramic substrates have been widely used in the electronic industry. For many years, ceramics have been metallized by high cost processes such as the ones using fused metal-glass pastes or by thin film vacuum deposition techniques. Attempts to reproducibly make circuit patterns by direct electroless deposition have not been successful due to poor adhesion of the metal films to the substrate and non-reproducible and non-uniform surface coverage.
Printed circuits on ceramics including alumina were described as early as 1947. See "Printed Circuit Techniques", National Bureau of Standards, Circular 468 (1947) and National Bureau of Standards, Misc. Pub. 192 (1948). One type, known as a thin film circuit, consists of a thin film of metal deposited on a ceramic substrate by one of the vacuum plating techniques. In these techniques, a chromium or molybdenum film, having a thickness of about 0.02 microns, acts as a bonding agent for copper or gold conductors. Photolithography is used to produce high resolution patterns etched from the thin metal film. Such conductive patterns may be electroplated, up to 7 microns thick. Due to their high cost, thin film circuits have been limited to specialized applications such as high frequency applications and military applications where a high pattern resolution is vital.
Another type of printed circuit, known as a thick film circuit, consists of circuit conductors composed of a metal and glass film fired on a ceramic substrate. Typically, the film has a thickness of about 15 microns. Thick film circuits have been widely used; they are produced by screen printing in a circuit pattern with a paste containing a conductive metal powder and a glass frit in an organic carrier. After printing, the ceramic parts are fired in a furnace to burn off the carrier, sinter the conductive metal particles and fuse the glass, thereby forming glass-metal particle conductors. The conductors are firmly bonded to the ceramic by the glass. Components may be attached to such conductors by soldering, wire bonding and the like.
Conductors in thick film circuits have only 30-60 percent of the conductivity of the respective pure metal. However, high conductivity of pure metal is needed to provide interconnections for high speed logic circuits. Because conductors in thick film circuits do not have such high conductivity, they do not provide optimum interconnections for high speed logic circuits.
The minimum conductor width and the minimum space between conductors which can be obtained by screen printing and firing under special high quality procedures is 125 and 200 microns, respectively. However, under normal production conditions, these minima are 200 and 250 microns, respectively.
Attempts have been made to directly bond pure metal conductors to ceramic substrates including alumina in order to achieve high conductivity for ceramic based circuit patterns. See U.S. Pat. No. 3,744,120, to Burgess et al. and U.S. Pat. No. 3,766,634 to Babcock et al. Solid State Technology 18/5, 42 (1975) and U.S. Pat. No. 3,994,430, to Cusano et al. describe a process for bonding copper sheets to alumina by heating the copper in air to form an oxide film on its surface. The treated copper sheet is bonded by the agency of this film to alumina at a temperature between 1065.degree. C. and 1075.degree. C. in a nitrogen furnace. In order to obtain well adhered copper foil without blisters: (1) the copper foil must be carefully oxidized to provide a black surface; (2) the copper oxide thickness must be carefully controlled; (3) the amount of oxygen in the copper foil must be controlled; (4) the oxygen content of the nitrogen furnace must be maintained at a controlled level to maintain a very moderately oxidizing atmosphere; and (5) the temperature must be controlled within one percent. This carefully controlled high temperature operation is difficult and expensive to tool for, to operate and to control. If the aforementioned extremely stringent controls are not maintained, blisters and other adhesion failures between the copper foil and the substrate are apparent. In spite of the difficult operating conditions, the process of Cusano et al. is being introduced into commercial application because of the need for the metallized product.
Although the above described systems are commercially used, the need for direct, simple metallization of ceramics with a layer or pattern of a pure metal conductor, such as copper, has prompted a continuous series of patents and proposed processes. See for example Apfelbach et al., Deutsches Patentschrift (DPS) No. 2,004,133; Jostan, DPS No. 2,453,192 and DPS No. 2,453,277; and Steiner DPS No. 2,533,524.
Other processes for producing printed circuit patterns on ceramic substrates are disclosed in U.S. Pat. Nos. 3,772,056; 3,772,078; 3,907,621; 3,925,578; 3,930,963; 3,959,547; 3,993,802 and 3,994,727. However, there is no teaching in all these patents of how to adhesion promote ceramic surfaces.
See also U.S. Pat. No. 3,296,012 to Stalnecker which discloses a process for producing a microporous surface for electrolessly plating alumina. Attempts to simply apply electroless metallization directly to ceramic substrates, have continually been tried and never been commercially successful. Toxic and corrosive materials such as hydrogen fluoride were tried to allow the direct bonding of electroless by formed metal deposit to ceramics without the use of firing temperatures. See, e.g., Ameen et al., J. Electrochem. Soc., 120, 1518 (1973). However, the hydrofluoric etch gave poor bond strength due to the resulting surface topography.
U.S. Pat. No. 4,428,986 to Schachameyer discloses a process for direct autocatalytic plating of a metal film on beryllia. The process comprises uniformly roughening the surface by immersing the beryllia in a 50% sodium hydroxide solution at 250.degree. C. for 7 to 20 minutes, rinsing with water, etching the beryllia with fluoboric acid for 5 to 20 minutes to attack the glass alloying constituents, rinsing with water, immersing the beryllia in a solution of 5 g/l stannous chloride and 3N hydrochloric acid, rinsing with water, followed by treating with 0.1 g/l palladium chloride solution, rinsing with water, and then electrolessly plating nickel on the beryllia. However, the etching step removes the silica and magnesium from the grain boundaries of the beryllia, thereby weakening the beryllia surface. As a result, the process of Schachameyer was able to achieve only 250 psi (1.7 MPa) bond strength before the beryllia substrate broke. This bond strength is only about a third of the bond strength normal in thick film type circuits and for many purposes not adequate.
U.S. Pat. No. 3,690,921 to Elmore discloses the application of a concentrated sodium hydroxide solution to the surface of a ceramic substrate. The ceramic substrate is heated to drive off the solvent (water) and is heated further to melt the sodium hydroxide and etch the ceramic surface. The molten sodium hydroxide has a tendency to coalesce on, and not uniformly wet, the ceramic surface. Smooth ceramic surfaces, e.g., having a surface roughness below 0.13 micrometers (5 microinches) are difficult to completely wet with molten sodium hydroxide. As a result, uneven etching of ceramic surfaces, particularly smooth ceramic surfaces, results with the use of molten sodium hydroxide. In the best cases, when a metal is subsequently bonded to the ceramic surface, the bond strength is uneven across the ceramic surface. In the worst case, there is no adhesion of metal in some areas of the ceramic surface, or even no metal deposit because there was no adhesion of the electroless plating catalyst. Thus, the process described by Elmore did not achieve commercial production.
U.S. application Ser. No. 502,748, filed June 9, 1983, "Metallization of Ceramics" by M. DeLuca et al., now abandoned discloses processes for improving surface coverage of electrolessly deposited metals by treatment of a ceramic surface by etching with a melt comprised of alkali metal compounds and contacting the surface with an acidic halide solution in a pre-treatment step immediately followed by, or constituting part of, the solutions employed in the catalyzing sequence for rendering the surface receptive to electroless deposition of metal. When the alkali metal compounds are applied to a very smooth ceramic surface, such as 99% alumina, either as aqueous solutions or as powder mixtures, as described in U.S. application Ser. No. 502,748, now abandoned coalescence occurs in the melt on the smooth ceramic surface.
All of the aforementioned processes for depositing metals on ceramic surfaces which include a etching step using alkali metal compounds in a molten state do not guarantee uniform adhesion promotion of the ceramic substrate.
The trend in ceramic printed circuit manufacturing is toward smoother and more uniform surface topography. A smooth surface provides better conductor definition and improved parameters for propogation of very high frequency signals at the substrate-conductor interface.
Unfortunately, the smoother the ceramic surface, the lower the net surface energy. As a result, the alkali metal compound does not completely wet such smooth ceramic surfaces having surface roughnesses of, e.g. 0.6 micrometers. During the fusion step, the liquid caustic tends to coalesce into one or more areas on the surface of the substrate to achieve lower net surface energy. This problem is greatest on the smooth surfaces of 99% pure electronic grade alumina. 89 to 96% alumina is somewhat easier to wet, although it frequently is difficult to achieve satisfactory results on a manufacturing scale. This results in a less than uniform surface etch and thus defective surface texture.
Total immersion of an alumina substrate in molten sodium hydroxides gives a uniform but severe surface etch. The severe surface etch results in a rough surface which does not permit fine conductor line resolution. In addition, such total immersion also tends to weaken the intrinsic structural integrity of the ceramic substrate resulting in cracks, especially in ceramic substrates provided with drilled holes.
As the purity of the ceramic increases, the surface also becomes smoother. Attempts to etch, for example, 99.5% pure electronic grade alumina by the procedures described in the Elmore U.S. Pat. No. 3,690,921, tend to result in a surface that is highly non-uniform.
Since 99.5% electronic grade alumina is normally used for microwave circuitry, surface roughness caused by deep etching must be avoided in order not to disturb the microwave signal propagation. However, it has not been possible to obtain a uniform, adherent metallization of smooth 99.5% alumina substrates by the procedures disclosed in Elmore U.S. Pat. No. 3,690,921 and/or DeLuca et al. U.S. application Ser. No. 502,748, filed June 9, 1983 now abandoned.
Non-uniform etching causes non-uniform adhesion of metal deposits, e.g., conductor-to-substrate and even areas of no adhesion or no conductor. It would be advantageous to be able to wet the ceramic more uniformly with an alkali metal compound, such as NaOH, and to be able to maintain uniform liquid-to-solid contact throughout the fusion process. This would result in a more consistent etching of ceramics and uniform adhesion of metal deposits formed on the surface of such ceramic substrates.